Catholic League, German Katholische Liga, a military alliance (1609–35) of the Catholic powers of Germany led by Maximilian I, duke of Bavaria, and designed to stem the growth of Protestantism in Germany. In alliance with the Habsburg emperors, the League’s forces, led by Johann Tserclaes, Graf von Tilly, played a key role in the Thirty Years’ War.

Plans for a league had long been discussed, but the formation of the Protestant Union in 1608 caused the Catholics to unite under Maximilian. The original League members on July 10, 1609, included Bavaria ; and the prince-bishops of Bavaria, Franconia, and Swabia; and the . The Rhenish ecclesiastical electors of Mainz, Trier, and Cologne joined on July 30 of that year. Maximilian reorganized the League in 1617, excluding the Rhenish members and making the League an exclusively southern German confederation.

The Bohemian revolt of 1618 caused alarm among the German Catholics, and the League resumed its expanded form in May 1619. When the Protestant elector Frederick V of the Palatinate accepted the Bohemian crown in 1619later that year, the Holy Roman emperor Emperor Ferdinand II looked to the League for military support. Through negotiations in 1620, Maximilian undermined the Protestant Union’s support of Frederick. In November 1620, the League’s forces under Tilly crushed Frederick at the Battle of White Mountain near Prague. Tilly’s troops then ravaged the Palatinate and other Protestant lands to the north; won several battles over King Christian IV of Denmark, who had come to the Protestants’ defense; and helped carry out Catholic restitutions in conquered Protestant territories. The importance of the League began to decline after 1626, when the Emperor found his own general in Albrecht von Wallenstein. Tilly’s defeat by Gustav II Adolf of Sweden at Breitenfeld in 1631, followed by his death the following year, accelerated the League’s decline. It was abolished in 1635 by the Peace of Prague, which forbade military confederations in the Empire.